A conventional voltage-controlled oscillator includes a constant current control circuit and a ring oscillator as disclosed exemplarily in JP H06-21776 A. In this voltage-controlled oscillator, the constant current control circuit receives a control voltage and outputs a current proportional to the control voltage to the ring oscillator.
The constant current control circuit includes an operational amplifier. The control voltage is applied to a non-inverting input terminal of an operational amplifier. An output terminal of the operational amplifier is connected to a gate terminal of an N-channel transistor. A resistor is provided between the N-channel transistor and the ground. A voltage between the N-channel transistor and the resistor is applied to an inverting input terminal of the operational amplifier.
According to this configuration, a current flowing in the resistor is determined by dividing a control voltage value by a resistance value of the resistor. The constant current control circuit thus outputs the current to the ring oscillator in proportion to the control voltage.
A conventional analog-digital converter exemplarily disclosed in JP H05-259907 A (U.S. Pat. No. 5,396,247) includes a circulation circuit, which is formed of plural delay elements connected in a ring form, and outputs as analog-digital conversion data a numeric value, which indicates a number of circulations of a signal through the circulation circuit and a position of circulation of the signal in the circulation circuit.
Since the constant current control circuit disclosed in JP H06-21776 A includes the operational amplifier, the output current tends to lose proportionality to the control voltage when the control voltage becomes a high frequency signal. When the output current of the constant current control circuit loses proportionality to the control voltage, an oscillation signal generated by the voltage-controlled oscillator also loses proportionality to the control voltage.
It is possible to produce the output current in proportion to the control voltage by driving transistors of the operational amplifier at high speeds, even when the control voltage is the high frequency signal. The transistors of the operational amplifier however consume more power when driven at high speeds.
The constant current control circuit is a single-end input type. Two current control circuits are needed to process a differential signal. The above-described circuit, which receives the control voltage and outputs the current proportional to the control voltage, is referred to as a voltage-current converter.